In the field of downlight fixtures, the heat dissipating efficiency of a downlight fixture may affect the lifetime and illumination of the downlight fixture. Conventionally, a heat sink formed by metallic material may have better heat dissipating efficiency. A conventional way to improve the heat dissipating efficiency is to install a downlight fixture into a metallic housing. However, the cost of the metallic housing is relatively high. Therefore, another conventional way is to replace the metallic housing with a plastic housing, in which the cost of the plastic housing is relatively low. In comparison to the metallic housing, the heat dissipating efficiency of the plastic housing is relatively low. Therefore, a plastic housing may not meet the requirement of a downlight fixture having high heat dissipation during the operation.